Interstellar objects may have seeded the formation of planets in solar systems like our own, potentially solving a key problem with planet formation theories.
In 2017, researchers observed an object from another solar system passing through our own for the first time. They named the object 鈥極umuamua, and the general view is that it was some sort of asteroid or comet ejected from its host star system. A second interstellar object, comet Borisov, was seen in 2019. The detection of 鈥極umuamua and Borisov suggests that there are many interstellar objects travelling around our galaxy at any given moment.
This further indicates that such objects could play a role during the birth of solar systems. The slow speed of young stars relative to their neighbours, coupled with the braking effect of the dust and gas that surround them, could cause these objects to enter orbit around a star rather than simply passing through like 鈥極umuamua and Borisov.
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and Colin Norman at the Space Telescope Science Institute in Baltimore, Maryland, modelled this process to estimate how many objects might become trapped by an average young star. They suggest that, over a period of maybe 10 million years, 600 billion objects about 1 metre in size would be captured, along with a further 200 million that were 10 metres wide, 60,000 that were 100 metres wide and 20 that were 1 kilometre wide.
鈥淲e were surprised that these numbers were pretty high,鈥� says Moro-Mart铆n. 鈥淏ut it is very uncertain, because we don鈥檛 really know how much material is out there.鈥�
The numbers suggest that interstellar objects could seed the birth of planets. These are thought to form by the accumulation of matter, gained either by accreting small, seed-like objects known as pebbles or through the collision of asteroid-like bodies known as planetesimals. But how you grow from dust in a disc to these larger objects is an open problem, known as the metre-size barrier.
鈥淲hen dust particles become larger, their collisions are more energetic,鈥� says Moro-Mart铆n. 鈥淲hen they collide, they start bouncing [off one another] rather than aggregating.鈥�
Interstellar objects could enable this material to condense, a little like the process through which dust in a cloud on Earth seeds raindrops. 鈥淭hese bigger bodies effectively act as condensation nuclei,鈥� says at the University of Canterbury in Christchurch, New Zealand, who wasn鈥檛 involved in the new analysis.
鈥淚t鈥檚 a feedback loop. At some point, you get one disc that had exactly the right conditions to make bigger things. You kick those into the interstellar population, and they get picked up in the next generation of planetary systems,鈥� she says.
This might indicate that we owe our very existence to material from another star, says at Queen鈥檚 University Belfast in the UK, who wasn鈥檛 involved in the study. 鈥淚t鈥檚 possible that Earth was started by an interstellar object sitting in the sun鈥檚 protoplanetary disc.鈥� The object would form only a tiny fraction of our planet鈥檚 bulk and almost certainly wouldn鈥檛 be detectable today.
Some of these trapped objects could remain in the outer reaches of our solar system, however, perhaps in the Kuiper belt or Oort cloud beyond Neptune. But identifying one would be 鈥減retty tough鈥�, says Fitzsimmons. 鈥淚t鈥檚 not clear how we would do that.鈥�
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